1. Field of the Invention
The present invention relates to a testing device for testing, for example, electrical characteristics of a portable device, and a calibration method employed therein.
2. Description of the Related Art
Conventionally, as this type of a testing device, a mobile-device testing system disclosed in, for example, patent document 1 (JP 2003-18104A), is known. The testing device described in patent document 1 comprises a signal generator for generating signals, an interface unit for switching the channel between the signal generator and a device as a measurement target, and a calibration power meter and a calibration receiver connected to the output side of the interface unit prior to testing in place of the measurement target device.
The conventional testing device constructed as the above measures, before execution of a test, the output of the interface unit channel by channel, using the calibration power meter and the calibration receiver, and pre-calculates calibration values associated with channel loss, thereby registering the values in a calibration table for correcting actual output signal levels.
By the way, in the conventional testing device, although calibration values can be relatively easily calculated if, for example, a measurement target device that has a single or a few antennas is tested, a great deal of time is required to test a recent technique, such as a carrier aggregation (CA) technique or a multiple-input multiple-output (MIMO) technique.
In the test of the CA or MIMO technique, a combining device for composing transmission signals and/or a distributor for distributing received signals is needed. The greater the number of combining devices or distributors, the greater the number of signal generators or the number of signal receivers.
The configuration of the interface unit on the transmission side for executing the test of the CA or MIMO technique are shown in FIGS. 13 to 15, assuming that the configuration of a conventional interface unit is used as a base. FIG. 13 is a view showing the configuration of an interface unit 110 that comprises signal generators 111a and 111b. FIG. 14 is a view showing the configuration of an interface unit 120 that comprises four signal generators 121a to 121d. FIG. 15 is a view showing the configuration of an interface unit 130 that comprises eight signal generators 131a to 131h. As is evident from FIGS. 13 to 15, the configuration of the interface unit becomes complex as the number of signal generators increases.
In FIGS. 13 to 15, interface units 110, 120 and 130 each comprise variable attenuators 112, switches SW 113, a combining device (or combining devices) 114 and an amplifier (or amplifiers) 115. Accordingly, if any of these configurations is adopted in the interface unit, significant signal level loss will occur, resulting in that the testing device does not satisfy test performance required by a device under test.
To overcome this problem, the following countermeasures may be taken:
(1) The signal generator is made to be able to output high power.
(2) The interface unit is made to comprise an amplifier.
(3) The interface unit is made to comprise an amplifier, and a level detector that enables the interface unit to perform auto level control (ALC), thereby enhancing the level accuracy of the output signal of the testing device.
Regarding item (1), the level loss of the interface unit can be compensated for if the signal generator outputs high power. However, the interface unit must have an amplifier excellent in linearity performance, which will raise problems, such as increases in required power and mounting area, and heat dissipation, resulting in an increase in device cost. Furthermore, the above-mentioned problem becomes more conspicuous in accordance with increases in the number of CAs or the number of MIMO antennas. In addition, since no consideration is given to a signal input to the testing device, there is a problem that the signal-to-noise ratio of the input signal will be degraded by a value corresponding to the level loss in the interface unit.
Regarding item (2), the level loss of the interface unit can be compensated for by an amplifier included in the interface unit. However, in general, there is a problem that the level accuracy of an output and an input will be degraded by fluctuation in gain due to fluctuation with time in the performance of the amplifier or due to fluctuation in temperature.
Regarding item (3), the level loss of the interface unit can be compensated for by the amplifier, and the level accuracy as the problem in item (2) can be solved by incorporating a level detector in the interface unit. In this case, however, the interface unit must comprise the level detector, which inevitably increases the device mounting area to thereby increase the device cost. This problem becomes serious in proportion to the number of signal generators.
As described above, in the conventional techniques, the time required for calibration cannot be shortened in testing, for example, the CA or MIMO technique, because a large number of signal generators and/or signal receivers are required to thereby make channels of transmitting signals complex.